1. Field of the Invention
The present invention relates to semiconductor devices with enhanced mobility.
2. Description of the Prior Art
Gallium arsenide (GaAs) is an appropriate material for high speed semiconductor devices, because electron mobility in GaAs is extremely high compared with that in silicon (Si). However, MOS (Metal-Oxide-Semiconductor) type field-effect-transistor corresponding to the Si-MOS transistor has not been realized in GaAs because of the difficulty in growing an insulating layer with good quality.
On the other hand, it has recently been known that a field-effect transistor can be fabricated by using aluminum gallium arsenide (AlGaAs) instead of an insulating layer to induce carriers at the GaAs-AlGaAs interfaces. Examples of such fabrication techniques are published in JAPANESE JOURNAL OF APPLIED PHYSICS, Vol. 19, No. 5, May, 1980, pp. L225-L227 and MICROWAVES, October, 1980, p. 20.
FIG. 1 shows the energy band diagram of the active region of this transistor using AlGaAs. 13, 12, and 11 denote the gate electrode, the doped AlGaAs layer, and the substantially undoped GaAs layer, respectively. F.sub.E designates the Fermi level. This figure illustrates the state where the carriers which are denoted by 15 are confined in a two-dimensional potential well parallel to the plane of the interface. The carriers 15 are provided by the donor levels 14 in AlGaAs layer 12, and transit in the undoped GaAs layer 11. Namely, the carriers are separated spacially from ionized donor impurities. Accordingly, the probability of carrier scattering by ionized impurity potential will be decreased, and as a result the mobility of the electron becomes high.
However, in the transistor of such structure with this high electron mobility, the transconductance becomes small, because the gate voltage is not effectively applied to the interface due to the large number of donors that are added.